Reverse osmosis
Water desalination
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Methods |
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Reverse
RO is most commonly known for its use in drinking water purification from seawater, removing the salt and other effluent materials from the water molecules.[2]
As of 2013 the world's largest RO desalination plant was in Sorek, Israel, outputting 624 thousand cubic metres per day (165 million US gallons per day).[3]
History
A process of osmosis through semi-permeable membranes was first observed in 1748 by
Osmosis
In (forward) osmosis, the solvent moves from an area of low solute concentration (high water potential), through a membrane, to an area of high solute concentration (low water potential). The driving force for the movement of the solvent is the reduction in the Gibbs free energy of the system in which the difference in solvent concentration between the sides of a membrane is reduced. This is called osmotic pressure. It reduces as the solvent moves into the more concentrated solution. Applying an external pressure to reverse the natural flow of pure solvent, thus, is reverse osmosis. The process is similar to other membrane technology applications.
RO differs from
RO requires pressure between 2–17
Membrane pore sizes vary from 0.1 to 5,000 nm. Particle filtration removes particles of 1 µm or larger. Microfiltration removes particles of 50 nm or larger. Ultrafiltration removes particles of roughly 3 nm or larger. Nanofiltration removes particles of 1 nm or larger. RO is in the final category of membrane filtration, hyperfiltration, and removes particles larger than 0.1 nm.[11]
Fresh water applications
This section needs additional citations for verification. (May 2023) |
Drinking water purification
Around the world, household drinking water purification systems, including a RO step, are commonly used for improving water for drinking and cooking.
Such systems typically include these steps:
- a sediment filter to trap particles, including rust and calcium carbonate
- a second sediment filter with smaller pores
- an activated carbon filter to trap organic chemicals and chlorine, which degrades certain types of thin-film composite membrane
- an RO thin-film composite membrane
- an ultraviolet lamp for sterilizing any microbes that survive RO
- a second carbon filter to capture chemicals that survive RO
In some systems, the carbon prefilter is replaced by a cellulose triacetate (CTA) membrane. CTA is a paper by-product membrane bonded to a synthetic layer that allows contact with chlorine in the water. These require a small amount of chlorine in the water source to prevent bacteria from forming on it. The typical rejection rate for CTA membranes is 85–95%.
The cellulose triacetate membrane rots unless protected by
Portable RO water processors are sold for personal water available. To work effectively, the water feeding to these units should be under pressure (typically 280 kPa (40 psi) or greater).[12] These processors can be used in areas lacking clean water.
US mineral water production uses RO. In Europe such processing of natural mineral water (as defined by a European directive)[13] is not allowed. In practice, a fraction of the living bacteria pass through RO through membrane imperfections or bypass the membrane entirely through leaks in seals.
For household purification absent the need to remove dissolved minerals (soften the water), the alternative to RO is an activated carbon filter with a microfiltration membrane.
Solar-powered RO
A
Sunlight's intermittent nature makes output prediction difficult without an energy storage capability. However batteries or thermal energy storage systems can provide power when the sun does not.[15]
Military
Larger scale reverse osmosis water purification units (ROWPU) exist for military use. These have been adopted by the
The water is treated with a polymer to initiate coagulation. Next, it is run through a multi-media filter where it undergoes primary treatment, removing turbidity. It is then pumped through a cartridge filter which is usually spiral-wound cotton. This process strips any particles larger than 5 µm and eliminates almost all turbidity.
The clarified water is then fed through a high-pressure piston pump into a series of RO vessels. 90.00–99.98% of the raw water's
Water and wastewater purification
RO-purified rainwater collected from storm drains is used for landscape irrigation and industrial cooling in Los Angeles and other cities.
In industry, RO removes minerals from
RO is used to clean effluent and
RO can be used for the production of
In 2002, Singapore announced that a process named NEWater would be a significant part of its water plans. RO would be used to treat wastewater before discharging the effluent into reservoirs.
Food industry
Reverse osmosis is a more economical way to concentrate liquids (such as fruit juices) than conventional heat-treatment. Concentration of orange and tomato juice has advantages including a lower operating cost and the ability to avoid heat-treatment, which makes it suitable for heat-sensitive substances such as protein and enzymes.
RO is used in the dairy industry to produce
Although RO was once avoided in the wine industry, it is now widespread. An estimated 60 RO machines were in use in Bordeaux, France, in 2002. Known users include many of elite firms, such as Château Léoville-Las Cases.
Maple syrup production
In 1946, some
Low-alcohol beer
When beer at typical concentration is subjected to reverse osmosis, both water and alcohol pass across the membrane more readily than other components, leaving a "beer concentrate". The concentrate is then diluted with fresh water to restore the non-volatile components to their original intensity.[18]
Hydrogen production
For small-scale hydrogen production, RO is sometimes used to prevent formation of mineral deposits on the surface of electrodes.
Aquariums
Many
Freshwater aquarists also use RO to duplicate the soft waters found in many tropical waters. While many tropical fish can survive in treated tap water, breeding can be impossible. Many aquatic shops sell containers of RO water for this purpose.
Window cleaning
An increasingly popular method of cleaning windows is the "water-fed pole" system. Instead of washing windows with conventional detergent, they are scrubbed with purified water, typically containing less than 10 ppm dissolved solids, using a brush on the end of a pole wielded from ground level. RO is commonly used to purify the water.
Landfill leachate purification
Treatment with RO is limited, resulting in low recoveries on high concentration (measured with
Power consumption for a disc tube module system
Energy consumption per m3 leachate | |||
---|---|---|---|
name of module | 1-stage up to 75 bar | 2-stage up to 75 bar | 3-stage up to 120 bar |
disc tube module | 6.1–8.1 kWh/m3 | 8.1–9.8 kWh/m3 | 11.2–14.3 kWh/m3 |
Desalination
Areas that have limited surface water or groundwater may choose to desalinate. RO is an increasingly common method, because of its relatively low energy consumption.[19]
Energy consumption is around 3 kWh/m3 (11,000 J/L), with the development of more efficient
Sea-water RO (SWRO) desalination requires around 3 kWh/m3, much higher than those required for other forms of water supply, including RO treatment of wastewater, at 0.1 to 1 kWh/m3. Up to 50% of the seawater input can be recovered as fresh water, though lower recovery rates may reduce membrane fouling and energy consumption.
Brackish water reverse osmosis (BWRO) is the desalination of water with less salt than seawater, usually from river estuaries or saline wells. The process is substantially the same as SWRO, but requires lower pressures and less energy.[1] Up to 80% of the feed water input can be recovered as fresh water, depending on feed salinity.
The Ashkelon desalination plant in Israel is the world's largest.[21][22][23]
The typical single-pass SWRO system consists of:
- Intake
- Pretreatment
- High-pressure pump (if not combined with energy recovery)
- Membrane assembly
- Energy recovery (if used)
- Remineralisation and pH adjustment
- Disinfection
- Alarm/control panel
Pretreatment
Pretreatment is important when working nanofiltration membranes due to their spiral-wound design. The material is engineered to allow one-way flow. The design does not allow for backpulsing with water or air agitation to scour its surface and remove accumulated solids. Since material cannot be removed from the membrane surface, it is susceptible to fouling (loss of production capacity). Therefore, pretreatment is a necessity for any RO or nanofiltration system. Pretreatment has four major components:
- Screening solids: Solids must be removed and the water treated to prevent membrane fouling by particle or biological growth, and reduce the risk of damage to high-pressure components.
- Cartridge filtration: String-wound polypropylene filters are typically used to remove particles of 1–5 µm diameter.
- Dosing: Oxidizing biocides, such as chlorine, are added to kill bacteria, followed by bisulfite dosing to deactivate the chlorine that can destroy a thin-film composite membrane. Biofouling inhibitors do not kill bacteria, while preventing them from growing slime on the membrane surface and plant walls.
- Prefiltration pH adjustment: If the pH, hardness and the alkalinity in the feedwater result in scaling while concentrated in the reject stream, acid is dosed to maintain carbonates in their soluble carbonic acid form.
- CO32− + H3O+ = HCO3− + H2O
- HCO3− + H3O+ = H2CO3 + H2O
- Carbonic acid cannot combine with calcium to form calcium carbonate scale. Calcium carbonate scaling tendency is estimated using the Langelier saturation index. Adding too much sulfuric acid to control carbonate scales may result in calcium sulfate, barium sulfate, or strontium sulfate scale formation on the membrane.
- Prefiltration antiscalants: Scale inhibitors (also known as antiscalants) prevent formation of more scales than acid, which can only prevent formation of calcium carbonate and calcium phosphate scales. In addition to inhibiting carbonate and phosphate scales, antiscalants inhibit sulfate and fluoride scales and disperse colloids and metal oxides. Despite claims that antiscalants can inhibit silica formation, no concrete evidence proves that silica polymerization is inhibited by antiscalants. Antiscalants can control acid-soluble scales at a fraction of the dosage required to control the same scale using sulfuric acid.[24]
- Some small-scale desalination units use 'beach wells'. These are usually drilled on the seashore. These intake facilities are relatively simple to build and the seawater they collect is pretreated via slow filtration through subsurface sand/seabed formations. Raw seawater collected using beach wells is often of better quality in terms of solids, silt, oil, grease, organic contamination, and microorganisms, compared to open seawater intakes. Beach intakes may also yield source water of lower salinity.
High pressure pump
The high pressure pump pushes water through the membrane. Typical pressures for brackish water range from 1.6 to 2.6 MPa (225 to 376 psi). In the case of seawater, they range from 5.5 to 8 MPa (800 to 1,180 psi). This requires substantial energy. Where energy recovery is used, part of the high pressure pump's work is done by the energy recovery device, reducing energy inputs.
Membrane assembly
The membrane assembly consists of a pressure vessel with a membrane that allows feedwater to be pushed against it. The membrane must be strong enough to withstand the pressure. RO membranes are made in a variety of configurations. The two most common are spiral-wound and hollow-fiber.
Only part of the water pumped onto the membrane passes through. The left-behind "concentrate" passes along the saline side of the membrane and flushes away the salt and other remnants. The percentage of desalinated water is the "recovery ratio". This varies with salinity and system design parameters: typically 20% for small seawater systems, 40% – 50% for larger seawater systems, and 80% – 85% for brackish water. The concentrate flow is typically 3 bar/50 psi less than the feed pressure, and thus retains much of the input energy.
The desalinated water purity is a function of the feed water salinity, membrane selection and recovery ratio. To achieve higher purity a second pass can be added which generally requires another pumping cycle. Purity expressed as total dissolved solids typically varies from 100 to 400 parts per million (ppm or mg/litre) on a seawater feed. A level of 500 ppm is generally the upper limit for drinking water, while the US Food and Drug Administration classifies mineral water as water containing at least 250 ppm.
Energy recovery
Energy recovery can reduce energy consumption by 50% or more. Much of the input energy can be recovered from the concentrate flow, and the increasing efficiency of energy recovery devices greatly reduces energy requirements. Devices used, in order of invention, are:
- Turbine or Pelton wheel: a water turbine driven by the concentrate flow, connected to the pump drive shaft provides part of the input power. Positive displacement axial piston motors have been used in place of turbines on smaller systems.
- Turbocharger: a water turbine driven by concentrate flow, directly connected to a centrifugal pump that boosts the output pressure, reducing the pressure needed from the pump and thereby its energy input,[25] similar in construction principle to car engine turbochargers.
- Pressure exchanger: using the pressurized concentrate flow, via direct contact or a piston, to pressurize part of the membrane feed flow to near concentrate flow pressure.[26] A boost pump then raises this pressure by typically 3 bar / 50 psi to the membrane feed pressure. This reduces flow needed from the high-pressure pump by an amount equal to the concentrate flow, typically 60%, and thereby its energy input. These are widely used on larger low-energy systems. They are capable of 3 kWh/m3 or less energy consumption.
- Energy-recovery pump: a reciprocating piston pump. The pressurized concentrate flow is applied to one side of each piston to help drive the membrane feed flow from the opposite side. These are the simplest energy recovery devices to apply, combining the high pressure pump and energy recovery in a single self-regulating unit. These are widely used on smaller low-energy systems. They are capable of 3 kWh/m3 or less energy consumption.
- Batch operation: RO systems run with a fixed volume of fluid (thermodynamically a closed system) do not suffer from wasted energy in the brine stream, as the energy to pressurize a virtually incompressible fluid (water) is negligible. Such systems have the potential to reach second-law efficiencies of 60%.[1][27][28]
Remineralisation and pH adjustment
The desalinated water is stabilized to protect downstream pipelines and storage, usually by adding lime or caustic soda to prevent corrosion of concrete-lined surfaces. Liming material is used to adjust pH between 6.8 and 8.1 to meet the potable water specifications, primarily for effective disinfection and for corrosion control. Remineralisation may be needed to replace minerals removed from the water by desalination, although this process has proved to be costly and inconvenient in order to meet mineral demand by humans and plants as found in typical freshwater. For instance water from Israel's national water carrier typically contains dissolved magnesium levels of 20 to 25 mg/liter, while water from the Ashkelon plant has no magnesium. Ashkelon water created magnesium-deficiency symptoms in crops, including tomatoes, basil, and flowers, and had to be remedied by fertilization. Israeli drinking water standards require a minimum calcium level of 20 mg/liter. Askelon's post-desalination treatment uses sulfuric acid to dissolve calcite (limestone), resulting in calcium concentrations of 40 to 46 mg/liter, lower than the 45 to 60 mg/liter found in typical Israeli fresh water.
Disinfection
Post-treatment disinfection provides secondary protection against compromised membranes and downstream problems. Disinfection by means of
Disadvantages
Large-scale industrial/municipal systems recover typically 75% to 80% of the feed water, or as high as 90%, because they can generate the required higher pressure.
Wastewater
Household RO units use a lot of water because they have low back pressure. Household RO water purifiers typically produce one liter of usable water and 3-25 liters of
Health
RO removes both harmful contaminants and desirable minerals. Some studies report some relation between long-term health effects and consumption of water low on calcium and magnesium, although these studies are of low quality.[32]
Waste-stream considerations
Depending upon the desired product, either the solvent or solute stream of RO will be waste. For food concentration applications, the concentrated solute stream is the product and the solvent stream is waste. For water treatment applications, the solvent stream is purified water and the solute stream is concentrated waste.
Research
This section needs to be updated.(March 2021) |
Improving Current Membranes
Current RO membranes, thin-film composite (TFC) polyamide membranes, are being studied to find ways of improving their permeability. Through new imaging methods, researchers were able to make 3D models of membranes and examine how water flowed through them. They found that TFC membranes with areas of low flow significantly decreased water permeability.[35] By ensuring uniformity of the membranes and allowing water to flow continuously without slowing down, membrane permeability could be improved by 30%-40%.[36]
Electrodialysis
Research has examined integrating RO with electrodialysis to improve recovery of valuable deionized products, or to reduce concentrate volumes.
Low-pressure High-recovery (LPHR)
Another approach is low-pressure high-recovery multistage RO (LPHR). It produces concentrated brine and freshwater by cycling the output repeatedly through a relatively porous membrane at relatively low pressure. Each cycle removes additional impurities. Once the output is relatively pure, it is sent through a conventional RO membrane at conventional pressure to complete the filtration step. LPHR was found to be economically feasible, recovering more than 70% with an OPD between 58 and 65 bar and leaving no more than 350 ppm TDS from a seawater feed with 35,000 ppm TDS.
Carbon Nanotubes (CNTs)
Carbon nanotubes are meant to potentially solve the typical tradeoff between the permeability and the selectivity of RO membranes. CNTs present many ideal characteristics including: mechanical strength, electron affinity, and also exhibiting flexibility during modification. By restructuring carbon nanotubes and coating or impregnating them with other chemical compounds, scientists can manufacture these membranes to have all of the most desirable traits. The hope with CNT membranes is to find a combination of high water permeability while also decreasing the amount of neutral solutes taken out of the water. This would help decrease energy costs and the cost of remineralization after purification through the membrane.[37]
Graphene
Graphene membranes are meant to take advantage of their thinness to increase efficiency. Graphene is a singular layer of carbon atoms, so it is about 1000 times thinner than existing membranes. Graphene membranes are around 100 nm thick while current membranes are about 100 µm. Many researchers were concerned with the durability of graphene and if it would be able to handle RO pressures. New research finds that depending on the substrate (a supporting layer that does no filtration and only provides structural support), graphene membranes can withstand 57MPa of pressure which is about 10 times the typical pressures for seawater RO.[38]
Batch RO may offer increased
The conventional approach claimed that molecules cross the membrane individually. A research team devised a "solution-friction" theory, claiming that molecules in groups through transient pores. Characterizing that process could guide membrane development. The accepted theory is that individual water molecules diffuse through the membrane, termed the "solution-diffusion" model.[39]
See also
- Electrodeionization
- ERDLator
- Forward osmosis
- Microfiltration
- Reverse osmosis plant
- Richard Stover, pioneered the development of an energy-recovery device currently in use in most seawater reverse-osmosis desalination plants
- Silt density index
- Salinity gradient
- Milli-Q water
- Water pollution
- Water quality
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{{cite book}}
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